A laboratory fume hood is a ventilated enclosure where harmful materials can be handled safely. The hood captures contaminants and prevents them from escaping into the laboratory by using an exhaust blower to draw air and contaminants in and around the hood's work area away from the operator so that inhalation of and contact with the contaminants are minimized. Access to the interior of the hood is through an opening which is closed with a sash which typically slides up and down to vary the opening into the hood.
The velocity of the air flow through the hood opening is called the face velocity. The more hazardous the material being handled, the higher the recommended face velocity, and guidelines have been established relating face velocity to toxicity.
When an operator is working in the hood, the sash is opened to allow free access to the materials inside. The sash may be opened partially or fully, depending on the operations to be performed in the hood. While fume hood and sash sizes vary, the opening provided by a fully opened sash is typically on the order of ten square feet. Thus the maximum air flow which the blower must provide is typically on the order of 750 to 1500 cubic feet per minute (CFM).
The sash may be closed when the hood is not being used by an operator. It is common to store hazardous materials inside the hood when the hood is not in use, and a positive airflow must be maintained to exhaust contaminants from such materials even when the hood is not in use and the sash is closed.
Additionally, it is highly desirable to maintain the air pressure in a fume hood laboratory slightly negative with respect to adjoining corridors or rooms. This ensures that any fumes which may escape from a fume hood do not go beyond the fume hood laboratory. This is advantageous both for safety reasons and also to avoid the spread of fumes which although not dangerous may be unpleasant to smell or breath. When the laboratory is kept at a slightly lower pressure than the adjoining rooms, any air flow through cracks or through doors which may be opened as people go in and out will be into the fume hood laboratory.
Until recently, most fume hood systems have exhausted a relatively constant volume of air. To maintain a more constant face velocity as the hood sash is moved up and down, so-called "by-pass" hoods have been developed. A by-pass hood has a by-pass opening through which air can enter the fume hood. The by-pass opening is blocked by the sash when it is in the fully opened position. As the sash is lowered, the by-pass opening is gradually uncovered so that air can "by-pass" the hood opening and enter the hood directly, thus preventing he air velocity through the hood opening from becoming too high as the sash is closed.
In many modern buildings, the air exhausted by a fume hood must be supplied by the heating, ventilation, and air conditioning (HVAC) system. To maintain the desired negative pressure in the laboratory, the make-up air supplied by the HVAC system should be slightly less than the amount of air exhausted by the hood. In fume hood systems exhausting a substantially constant volume of air, this may be easily done by setting the make-up air volume to be slightly less than the sum of the fume hood exhaust air volume and the air removed and recirculated by the HVAC system.
During hot summer months and cold winter months, the costs associated with heating and cooling the air exhausted from the laboratory by the fume hood can be substantial. With the recent increases in the costs of energy, new methods of controlling the air exhausted by a fume hood have been developed which reduce the amounts of air exhausted. Additionally, more stringent safety regulations have put stricter requirements on the variation in face velocity which can be tolerated, and these requirements may be met by varying the total amount of air exhausted by the hood. An example of such a system is described in U.S. Pat. No. 4,528,898 for a Fume Hood Controller, and the material in this patent is incorporated by reference herein. Another example is described in U.S. Pat. No. 4,706,553 and incorporated by reference herein.
Fume hood control systems such as those described in the above-referenced patent and application result in variable amounts of air being exhausted from the laboratory. In buildings were the air must be supplied by the (HVAC) system, the volume of make-up air must vary as the fume hood exhaust air volume changes in order to maintain a negative pressure level in the fume hood laboratory
One previous approach to this problem includes the use of a static pressure sensor to measure the difference between the laboratory pressure and the pressure in the adjoining corridor Make-up air is varied in response to the output from the sensor to maintain a constant pressure differential. There are several limitations to this procedure. First, it may be difficult to implement where the fume hood room opens into several different parts of the building. Typically, the pressure differentials between several rooms varies as the HVAC system increases and decreases air flow to different areas of the building. Maintaining a slightly negative pressure with respect to one adjoining room does not ensure that the laboratory pressure will always be negative with respect to the other rooms.
Additionally, the impedance or porosity between the laboratory and the adjoining corridor or rooms is a function of the area of the openings therebetween and may suddenly change by a large amount. For example a 100 cubic feet per minute (CFM) air flow into the laboratory may suddenly increase to 2000 CFM for a given pressure differential when a door is opened. This is undesirable. In addition to possibly creating annoying "breezes" through the door or other openings, the sudden increase in air flowing into the laboratory may tax the air supply into the corridor and may disturb the air balances of the HVAC system.
More importantly, it is difficult to ensure adequate performance in many installations due to the trade-off which must be made between the response time of the make-up air and the stability of the HVAC system. The air pressure differential which must be sensed is very small, typically on the order of 0.02 inches of water. Such small pressure differentials are difficult to sense and will contain a lot of "noise" caused by small variations in the pressure drops through the ducting and other parts of the HVAC system as the HVAC system changes air flows to various other parts of the building during normal operation. Long time lags and filters must typically be used to smooth out these variations. In some installations, large oscillations in air flows may result from changes in the system, such as when a door is opened, which in the worst case may become unstable, if such filtering is not used.
Another previous approach uses a switch located near the hood which selects between a low and high flow level. The switch may be operated manually or be activated by the movement of the hood sash. The make-up air supply also receives the signal from the switch and uses this signal to maintain a constant differential between the exhausted air and the make-up air. For example, if the hood exhaust volume may be switched between 500 CFM and 1,000 CFM, a damper controlling the make-up air volume would be switched between two positions corresponding to 300 CFM and 800 CFM. The maximum turndown ratio for such a system, however, is limited. Turndown ratios greater than two-to-one can produce variations in face velocity that may make the hood unsafe to use.